Add torch.ops.nvprims namespace for nvFuser-specific prims (#82155)
New namespace `torch.ops.nvprims` is meant for specific to the nvFuser set of primitives. All `impl_nvfuser` attributes are removed from `torch.ops.prims` functions.
`NvfuserPrimsMode()` context manager can be used for automatic rewrite of `torch.ops.prims` calls to `torch.ops.nvprims` when possible.
The previous way to test whether a prim would be executable with nvFuser was to test `impl_nvfuser is not None`, now all functions in the `torch.ops.nvprims` namespace are supposed to have the `impl_nvfuser` attribute and hence all are executable by nvFuser.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82155
Approved by: https://github.com/jjsjann123, https://github.com/ngimel
